Laser surgery is a technique which is finding increasing use in a number of situations. Physicians today routinely use lasers in a number of specialized procedures, including reshaping the cornea of the eye and ablating disfiguring skin lesions such as port wine stains. Lasers are used for delicate surgical microdissection in the brain, but are also capable of blasting apart obstructing kidney and gallbladder stones, removing calcified arteriosclerotic plaque from obstructed blood vessels, and destroying tumors.
Another major surgical use of lasers is in retinal photocoagulation, which is used to treat diabetic patients suffering from diabetic retinopathy. There are approximately 7,000,000 diabetics in the United States today (approximately 2.8% of the population), and of these, approximately 700,000 have retinal complications. Diabetic hyperglycemia is often the triggering factor for these complications. It leads to retinal hypoxia which in turn stimulates the production of the vasogenic factor which causes neovascularization. About one tenth of the patients having such retinal complications are candidates for laser photocoagulation treatment. The general purpose of such treatment is to destroy the areas of neovascularization, and to preserve other areas of the retina.
Retinal photocoagulation can be divided into several categories of treatment. Local photocoagulation treatment consists of confluent laser burns applied to the specific neovascularization and a 500 .mu. area around it. Focal photocoagulation treatment consists of single laser burns applied to isolated vascular lesions which are near the fovea and thus threaten central vision. Panretinal photocoagulation consists of 1500 laser burns which literally ablate the majority of peripheral retina tissue, but are intended to leave the fovea and parafoveal region intact. Thus, the hypoxic peripheral retina is sacrificed along with a large portion of the peripheral visual field and night vision in order to decrease the production of vasogenic factor and hopefully save the fovea and central vision.
Another use of laser photocoagulation is to repair retinal tears and retinal detachment. The neural retina is composed of a thin layer of neurons which hangs together as a translucent sheet and overlays supportive tissue of the eye ball. The retina is physically attached to the other structures of the eye in two regions: the area of the optic disc near the center of the retina, and in the area of the ora serrata which forms the peripheral edge of the retina. Under normal conditions, these points of attachment serve to keep the retina flattened snugly against the back of the eyeball. In cases of trauma to the eye, intraretinal bleeding (such as occurs with diabetes), processes associated with "normal" aging, and in some cases where etiology is unknown, the retina does not maintain its flattened position. The retina can become too tightly stretched and small tears in the retinal tissue can occur; the retinal surface can become ruffled, or small blobs can form, causing the retina to become detached from the underlying tissue. Left unattended, small areas of disruption can lead to much larger areas of detachment. Areas of detached retina do not maintain normal function and may actually undergo necrosis. These areas are routinely treated with lasers in order to produce photocoagulated tissue which will act to re-bond the retina to underlying tissue. Vision in this region is lost, but further retinal detachment is prevented.
Laser surgery in general has a number of advantages, such as permitting operations with smaller exposure, more precise control of what tissue is eliminated, reduced blood loss during surgery, reduced postoperative edema, shorter operative time, and potentially reduced operative mortality and morbidity and increased longevity. However, laser surgery also has some disadvantageous side effects For example, when lasers are used to treat neuronal tissue such as the retina, the damage resulting from the laser burn usually spreads into surrounding healthy tissue. The size of lesions caused by laser burns in retinal tissue increases in the days immediately following the laser treatment. There are several mechanisms for the damage caused by lasers.
Photothermal effects are caused by transformation of absorbed light energy into heat. Depending on the amount of heat generated, tissues are (1) destroyed by vaporization and/or combustion, (2) carbonized, or (3) coagulated. A second category, photochemical effects, result from photoactivation of certain exogenous photosensitizers selectively sequestered in various tissues, such as cancerous growths, to produce toxic substances that destroy the tissue or lesion. The actual mechanism of action is not well understood; however, in some cases it involves the light activation of photosensitizers which interact with molecular oxygen to form singlet oxygen, a strong oxidant, which in turn causes oxidation of vital cell constituents. A third category consists of photoacoustic mechanical effects which involve the rapid heating and expansion of the target tissue. This creates an explosive shock wave that may disrupt, fragment, or ablate cells, organelles and extracellular matrix in the absence of overt thermal or chemical reactions.
The present invention stems from the belief that, in addition to the above-listed mechanisms, glutamate may play a role in causing damage to tissue surrounding a laser burn.
Glutamate is a highly abundant compound in the nervous system, where it serves a number of diverse functions. As an amino acid, it serves as one of the building blocks for the synthesis of various proteins; as a metabolite in the tricarboxylic acid cycle, it is important in energy metabolism; it is the principal excitatory neurotransmitter used in the central nervous system; and it is used to a lesser extent as a transmitter in the peripheral nervous system. When glutamate is released from intracellular stores into the extracellular space, it has strong depolarizing actions on most neurons with which it comes in contact. Several cellular systems operate to restrict or "buffer" the normal level of glutamate found extracellularly. High affinity uptake systems for glutamate are found on glutamatergic neurons and they serve to recycle the released glutamate. Similar uptake systems are located on glial cells which accumulate and degrade glutamate into its inactive metabolite, glutamine. Virtually any cellular insult or injury in the nervous system, including a laser burn, can lead either directly or indirectly to a loss of the integrity of the intracellular storage pools of glutamate, a breakdown in glutamate uptake activity and an interruption of glutamate metabolism. The net result is an increase in extracellular glutamate, massive depolarization or excitation of surrounding neurons and, in many cases, widespread cell death. Thus glutamate excitotoxicity may mediate cell death associated with a wide variety of primary neuronal injuries and may represent one of the final common pathways for cell death in the brain and other parts of the nervous system.
The actions of extracellular glutamate are believed to be brought about by the binding of glutamate to one of four different types of membrane receptors which are found on most neurons. One specific type of glutamate receptor is thought to be a quiescent or silent receptor during normal housekeeping types of neuronal activity, but may be called into play or activated during specific neuronal processing events such as learning or memory recall. This receptor is named after the glutamate analogue for which it has highest affinity, N-methyl-D-aspartate (NMDA). Magnesium normally binds the NMDA receptor, maintaining it in an inactive state. Two signals lead to activation of the receptor, namely a change in membrane potential (depolarization) or an increase in calcium. These changes occur during repetitious neuronal firing which may associated with the types of neuronal activity expressed during a learning trial. The displacement of magnesium from the NMDA receptor allows glutamate to bind to the receptor and open a high conductance ionophore, leading to additional influx of sodium and potassium, and further depolarization. Thus the initial activation of a neuron by glutamate through non-NMDA receptors or by some other excitatory neurotransmitter will make the cell more sensitive to any subsequent exposure to extracellular glutamate.
This unique property of the NMDA type of glutamate receptor is believed to make it a likely candidate for participation in glutamate induced cell death. Glutamate, released in response to some initial cellular injury, would make surrounding neurons even more susceptible to glutamate depolarization through NMDA receptors, with a massive influx of sodium and potassium, leading to ionic imbalances, eventual cell death and release of more glutamate. Because of the domino effect, the initial site of injury would be increased many fold because of spreading secondary damage caused by glutamate interaction with NMDA receptors.
As noted above, laser surgery has a number of benefits, but it also has the disadvantage when used on neuronal tissue that healthy tissue surrounding the area of the burn is usually damaged soon after the laser treatment. The present invention provides means of reducing this damage, and therefore makes the laser treatment even more desireable.